Wireless communication systems that are provided with a plurality of channels such as voice channels or control channels to provide communication services to various wireless devices are known (for example, Japanese Patent Publication No. 2724917).
Now, from the perspective of improving the efficiency of use of wireless frequency bands, bands of communication paths used by wireless devices are becoming narrower. For example, part 90 of the CFR (Code of Federal Regulation) 47 defined by the FCC (Federal Communications Commission) of the U.S. makes regulations on certain wireless devices so that one communication path should be provided per 6.25 kHz from the year 2005. Then, wireless devices that can not meet this requirement will not be able to receive type approval from 2005 and will not be allowed to be sold inside the U.S.
In the case of the wireless devices that operate at this bandwidth of 6.25 kHz, the modulation spectrum at the transmitter is required to be in the range of a transmit spectral mask called “Mask E” as illustrated in FIG. 5. The restrictions imposed by the transmit spectral mask defined in this “Mask E”, provide, for example: that at the frequency ±3 kHz apart from the carrier, the transmit spectrum should be below −30 dB; that in the case of wireless devices having transmission power of 10 W or more, at the frequency ±4.6 kHz apart from the carrier, the transmission spectrum should be below −65 dB; and that in the range between ±3 kHz and ±4.6 kHz from the carrier, the transmit spectrum should be below the straight line passing through −30 dB at ±3 kHz and −55 dB at ±4.5 kHz.
Conventionally, wireless devices operating at 12.5 kHz bandwidth or 25 kHz bandwidth adopt the analog FM scheme in which a carrier is directly frequency-modulated (FM) with speech signals and transmitted. However, for the matching to the transmit spectral mask defined in “Mask E” as illustrated in FIG. 5, it is required to substantially decrease the degree of frequency-modulation. Consequently, with the conventionally-used analog FM scheme, the S/N ratio is significantly degraded so that performance sufficient for practical use cannot be obtained. Therefore, in order to achieve both the matching to the transmit spectral mask and the sufficient performance, it is considered that transmitting four-level. FSK signals is performed that is generated by converting all speech signals and control signals to digital signals and then FSK (Frequency Shift keying) modulating a carrier therewith.
FIG. 6 illustrates an example of a spectral distribution of a four-level FSK signal that is matched to the transmit spectral mask of “Mask E”. As illustrated in FIG. 6, it is possible to match a four-level FSK signal to the transmit spectral mask of “Mask E” in the given experimental environment. Referring to FIG. 6, the power level of adjacent channels detuned by 6.25 kHz is the noise floor level. In this regard, the leak of power to the adjacent channels also seems to be sufficiently suppressed.
However, since the FSK modulation, unlike amplitude modulation (AM), is non-linear, it has the tendency of broad transmit spectrum as compared with amplitude modulation. Also, when the bandwidth of a filter (BPF: Band Pass Filter) provided in a transmitter is ±2 kHz in an actual use environment, the spectral components which extend over and from the range of Δ4.25 kHz to the range of Δ8.25 kHz (the ranges have the center point at the transmit frequency) may have affections as leakage power on adjacent channels. The property illustrated in FIG. 6 also shows there exist spectral components of the four-level FSK signal beyond the range of Δ4.25 kHz having the center point at the transmit frequency. Therefore, the level of interference with adjacent channels of wireless devices operating at 6.25 kHz bandwidth is greater than even the level of interference with adjacent channels of wireless devices operating at the conventional 12.5 kHz bandwidth. Due to such characteristics of the four-level FSK signal, a voice call conducted using a wireless device with the four-level FSK modulation scheme may cause adjacent channel interference to another wireless device that is conducting a call using an adjacent call channel.
Moreover, since the FSK modulation employs modulation signals generated by digitizing speech signals and control signals, even when sound to be transmitted is absent, speech data corresponding to the absence of sound are outputted from an speech encoding circuit. This results in the FSK signal always having a certain transmit spectral extent. On the other hand, in the analog FM scheme, which is adopted by conventional wireless devices operating at the 12.5 kHz bandwidth, a carrier is directly frequency-modulated with speech signals, so that the absence of sound only results in the output of an almost non-modulated carrier with the transmit spectrum limited to a narrower range than that for the presence of sound. In addition, considering that an actual conversation involves numerous moments where sound pressure decreases at the boundaries between words or clauses, the average degree of frequency modulation (frequency-shift keying) in the analog FM scheme may be substantially smaller than the maximum modulation degree.
Thus, since the four-level FSK modulation scheme have different characteristics from those of the Analog FM scheme, when comparing for the degrees of interference with adjacent channels, the advantage brought by the use of four-level FSK signals could be less in the case of an call of an actual conversation than the case of measuring in an given experimental environment with a fixed frequency modulation.
In light of the above-mentioned circumstances, an object of the present invention is to provide a wireless communication apparatus and a wireless communication method that are suitable for narrowed bands of communication paths and can reduce the affections on adjacent channels.